Installation, Wiring,
and Specifications
2–18
Installation, Wiring, and Specifications
In the next example we connect a PLC DC output point to the sinking input of a field
device. This is a bit tricky, because both the PLC output and field device input are
sinking type. Since the circuit must have one sourcing and one sinking device, we
add sourcing capability to the PLC output by using a pull-up resistor. In the circuit
below, we connect R
pull-up from the output to the DC output circuit power input.
Field Device
Output
Ground
Input
Common
PLC DC Output
+DC pwr
+
–
(sourcing)
(sinking)
Power
(sinking)
pull-up
Supply
R
input
R
NOTE: DO NOT attempt to drive a heavy load (>25 mA) with this pull-up method.
NOTE 2: Using the pull-up resistor to implement a sourcing output has the effect of
inverting the output point logic. In other words, the field device input is energized
when the PLC output is OFF, from a ladder logic point-of-view. Your ladder program
must comprehend this and generate an inverted output. Or, you may choose to
cancel the effect of the inversion elsewhere, such as in the field device.
It is important to choose the correct value of R pull-up. In order to do so, we need to
know the nominal input current to the field device (I input) when the input is energized.
If this value is not known, it can be calculated as shown (a typical value is 15 mA).
Then use I
input and the voltage of the external supply to compute R pull-up. Then
calculate the power Ppull-up (in watts), in order to size R pull-up properly.
pull-up
R
input
R
=
supply
V
– 0.7
–
input
I
input
I
=
input (turn–on)
V
input
R
pull-up
P
=
supply
V
2
pullup
R
The drawing below shows the actual wiring of the DL05 Micro PLC to the supply and
pull-up resistor.
+
–
Output
Common
Supply
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